In perpendicular magnetic recording, a (magnetically) soft underlayer (SUL) is essential for enhancing the write field through the imaging effect of the main write pole. However, it also enhances stray fields at undesired locations, such as the corners of the reader/writer shields. This causes a shield erasure problem when the hard drive is subject to an external field. Efforts have been made to improve the external field robustness (EFR) for PMR head by modifying the shapes of the shields.
A simple example is shown in FIG. 1 where the main body 10 of the shield has been under-cut at its lower corner at a shallow angle 11 (from US 2006/0245114 A1 FIG. 5). Also shown in FIG. 1 is recording media 13. This shape is being used in today's products.
Another method involves extending tabs out from the sides of the shield. These could each have an end with a downward triangular shape (21 in FIG. 2a), a rectangular shape (22 in FIG. 2b), or with a shallow cut in the tab, as shown in FIG. 2c. Among the three shapes shown in FIG. 2, the tab with shallow cut at the ABS (air bearing surface) has the best EFR performance, as the other two have sharper corner ending at the ABS, which will cause more severe local charge build-up and thus higher erasure field. However, as shown below by finite element modeling (FEM), even the shallow cut tabbed design still has its limitations.
FIG. 3 shows FEM results for the tab with shallow cut under a 400 Oe external vertical field. For very thin tab thickness of 0.2 μm (FIG. 3a), demagnetization effects force the flux to stay in plane, thus eliminating the hot spot at the shallow cut corner (point B). There is, however, significant flux crowding and charge build-up at the corner (point A) where the tab meets the shield. As a result, this corner becomes the new erasure spot. On the other hand, FEM further shows that when the tab thickness is large (2 μm for FIG. 3b), the flux crowding at corner A is relieved, but the hot spot moves back to corner B due to the weakened demagnetization effect from the thicker tab.
FIG. 4 displays the maximum vertical field in the media at the two corners for different tab thicknesses. It clearly shows how the hot spot moves from A to B as the tab thickness increases, as a result of the two competing mechanisms. Since it is the maximum field among all locations that counts, FIG. 5 plots the maximum field (the greater of the two corner fields in FIG. 4 for each case) vs. the tab thickness, compared to a reference case, which is a standard shallow cut (FIG. 1 shape) with the same cut angle of 10 degree. This shows that only at a tab thickness of 1.0 μm is significant erasure field reduction vs. reference achieved. For tab thickness of 0.4 μm, or below, it is even worse than the reference.
These examples make it clear that improved EFR performance beyond what is available in the prior art is needed. This is the objective of the present invention.
The following patent applications describe several methods for improving EFR:
US patent application No. US 2006/0245113 A1 (Lijie Guan, Moris Dovek), and US patent application No. US 2006/0245114 A1 (Lijie Guan, Moris Dovek). A routine search of the prior art was also performed, with the following additional references of interest being found:
U.S. Pat. No. 7,460,342 (Guan et al-Headway) shows tapered shield edges while U.S. Patent Application 2005/0219747 (Hsu et al) shows a tapered trailing shield.